[FLINK-8333] [flip6] Separate deployment options from command options

This commit separates the parsing of command options and deployment options into two
steps. This makes it easier to make the CustomCommandLines non-static.

Moreover, this commit moves the CliFrontend into the cli sub package.

flink-clients/src/main/java/org/apache/flink/client/cli/CliArgsException.java

@@ -28,4 +28,8 @@ public class CliArgsException extends Exception {
 	public CliArgsException(String message) {
 		super(message);
 	}
+
+	public CliArgsException(String message, Throwable cause) {
+		super(message, cause);
+	}
 }